The present invention relates to a novel process for xe2x80x9ccontrolledxe2x80x9d or xe2x80x9clivingxe2x80x9d free-radical polymerization, giving access to block copolymers.
Block polymers are usually prepared by ionic polymerization. A disadvantage of this type of polymerization is that it permits the polymerization only of certain types of non-polar monomers, particularly styrene and butadiene, and that it requires a particularly pure reaction environment, and temperatures often lower than ambient, in order to minimize side reactions, and the result is severe operational constraints.
An advantage of free-radical polymerization is that it is easy to implement without adhering to excessive purity requirements, and at temperatures of ambient or above. However, until recently there was no free-radical polymerization process which could give block polymers.
A novel process for free-radical polymerization has now been developed: this is what is known as xe2x80x9ccontrolledxe2x80x9d or xe2x80x9clivingxe2x80x9d free-radical polymerization. Controlled free-radical polymerization proceeds by growth through propagation of macroradicals. These macroradicals have a very short lifetime and recombine irreversibly via coupling or dismutation. When the polymerization proceeds in the presence of a number of comonomers, the variation in the composition of the mixture is infinitely slow compared with the lifetime of the macroradical, and therefore the chains have a random sequence of monomer units, rather than a block-type sequence.
In recently developed techniques for controlled free-radical polymerization, the extremities of polymer chains can be reactivated as a radical by homolytic cleavage of a bond (for example Cxe2x80x94O or C-halogen).
Controlled free-radical polymerization therefore has the following distinctive aspects:
1. the number of chains is fixed for the entire duration of the reaction,
2. all the chains grow at the same rate, resulting in:
linear increase in molecular mass with conversion,
a narrow distribution of masses,
3. the average molecular mass is controlled by the molar ratio monomer/chain precursor,
4. the possibility of preparing block copolymers.
The controlled character is all the more pronounced if the rate of reactivation of the free-radical chains is very great compared with the rate of growth of the chains (propagation). There are cases where this does not always apply (i.e. the reactivation rate of the free-radical chains is greater than or equal to the rate of propagation) and conditions 1 and 2 are not complied with, but it is nevertheless still possible to prepare block copolymers.
The publication WO 98/58974 describes a living free-radical polymerization process giving access to block copolymers by a process without UV irradiation, by using xanthate compounds, i.e. compounds having the function: 
This free-radical polymerization allows preparation of block polymers with the aid of any kind of monomer, without any UV source. The polymers obtained do not contain any metallic impurities detrimental to their use. They have chain-end functionalization and a low polydispersity index, lower than 2, or even lower than 1.5.
It is an object of the present invention to propose a novel procedure for polymerization with the aid of new precursors of xanthate type.
Another object is to propose a polymerization process which uses precursors of xanthate type and during the course of which the number-average molar masses Mn of the resultant polymers are well controlled, i.e. close to the theoretical values Mn th, especially at the start of the polymerization reaction.
Another object is to propose a polymerization process which uses precursors of xanthate type to synthesize polymers and block copolymers whose index of polydispersity (Mw/Mn) is low, i.e. close to 1.
With this object in mind, the invention provides a process for preparing polymers, characterized by bringing into contact:
at least one ethylenically unsaturated monomer,
at least one source of free radicals, and
at least one compound (I) of general formula (IA), (IB), or (IC) 
in which:
R2 and R2, represent:
an alkyl, acyl, aryl, alkene, or alkyne group (i), or
a carbocyclic system (ii), saturated or unsaturated, optionally aromatic, or
a heterocyclic system (iii), saturated or unsaturated,
these groups and cyclic systems (i), (ii), and (iii) being substituted by at least one fluorine atom, chlorine atom, and/or bromine atom,
R1 and R1xe2x80x2 represent:
an alkyl, acyl, aryl, alkene, or alkyne group (i), optionally substituted, or
a carbocyclic system (ii), saturated or unsaturated, optionally substituted or aromatic, or
a heterocyclic system (iii), saturated or unsaturated, optionally substituted, where these groups and cyclic systems (i), (ii) and (iii) may be substituted by substituted phenyl groups, substituted aromatic groups, or: alkoxycarbonyl or aryloxycarbonyl (xe2x80x94COOR), carboxy (xe2x80x94COOH), acyloxy (xe2x80x94O2CR), carbamoyl (xe2x80x94CONR2), cyano (xe2x80x94CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (xe2x80x94OH), amino (xe2x80x94NR2), halogen, allyl, epoxy, alkoxy (xe2x80x94OR), S-alkyl, or S-aryl groups, groups having hydrophilic or ionic character, for example the alkali metal salts of carboxylic acids, the alkali metal salts of a sulfonic acid, polyalkylene oxide chains (PEO, PPO), or cationic substituents (quaternary ammonium salts), R representing an alkyl or aryl group, or
a polymer chain,
is between 2 and 10.
The process according to the invention therefore consists in bringing into contact a source of free radicals, an ethylenically unsaturated monomer, and a compound (I) of formula (IA), (IB), or (IC).
This compound (I) bears a xanthate functionality. According to the essential characteristic of the invention, the xanthate functionality bears a group R2 or R2xe2x80x2 which has to be substituted by at least one fluorine atom, chlorine atom, and/or bromine atom. R2 and R2xe2x80x2 are preferably substituted by at least one fluorine atom, and still more preferably only by fluorine atoms.
According to one preferred version, R2 represents a group of formula: xe2x80x94CH2Rxe2x80x25, in which Rxe2x80x25 represents an alkyl group substituted by at least one fluorine atom, chlorine atom, and/or bromine atom. According to this embodiment, preferred groups R2 are the following:
xe2x80x94CH2CF3,
xe2x80x94CH2CF2CF2CF3 
xe2x80x94CH2CH2C6F13,
According to another preferred version, R2 represents the group CH(CF3)2.
R1 in the formulae (IA) and (IB) preferably represents:
a group of formula CRxe2x80x21Rxe2x80x22Rxe2x80x23, in which:.
a Rxe2x80x21, R2 and Rxe2x80x23 represent the groups (i), (ii), or (iii) as defined above, or
Rxe2x80x21=R=2=H and Rxe2x80x23 is an aryl, alkene, or alkyne group,
or a group of formula xe2x80x94CORxe2x80x24 in which Rxe2x80x24 represents a group (i), (ii), or (iii) as defined above.
The most interesting results have been obtained for the compound (I) when R1 is a group selected among:
xe2x80x94CH(CH3) (CO2Et)
xe2x80x94CH(CH3) (C6H5)
xe2x80x94CH(CO2Et)2 
xe2x80x94C(CH3) (CO2Et) (Sxe2x80x94C6H5)
xe2x80x94C(CH3)2(C6H5) 
in which Et represents an ethyl group and Ph represents a phenyl group.
The groups R1 and R1xe2x80x2 may also represent a polymer chain from a free-radical or ionic polymerization, or from a polycondensation. Preferred compounds of formula (IC) are those for which R1xe2x80x2 is the group xe2x80x94CH2xe2x80x94 phenyl xe2x80x94CH2xe2x80x94 or the group xe2x80x94CHCH3CO2CH2CH2CO2CHCH3xe2x80x94.
In the preferred embodiment of the invention, the polymerization process uses a compound (I) formula (IA). Preferred compounds of formula (IA) are ethyl a-(O-heptafluorobutylxanthyl)propionate (R1xe2x95x90CHCH3 (CO2Et), R2xe2x95x90CH2CF2CF2CF3), ethyl a-(O-trifluoroethylxanthyl)propionate (R1xe2x95x90CHCH3(CO2Et), R2xe2x95x90CH2CF3), and ethyl ethyl a-(O-tridecafluorooctanylxanthyl)propionate (R1xe2x95x90CHCH3(CO2Et), R2xe2x95x90CH2CH2C6F13)
The compounds of formulae (IA), (IB), and (IC) are easily accessible. They may particularly be obtained by reacting an alcohol R2OH with carbon disulfide CS2 (in the presence of hydrogen hydride, for example), giving the xanthate R2O(Cxe2x95x90S)Sxe2x88x92Na+. This xanthate is then reacted with an alkyl halide R1X (X=halogen), giving the halogenated xanthate: Rxe2x80x2O (Cxe2x95x90S)xe2x80x94SR1.
According to the process of the invention, the free-radical source is generally a free-radical polymerization initiator. However, for certain monomers, such as styrene, thermal initiation is sufficient to generate free radicals.
In the first case, the free-radical polymerization initiator may be selected be selected among conventional initiators used in free-radical polymerization, for example one of the following initiators:
hydrogen peroxides, such as: tert-butyl hydroperoxide, cumene hydroperoxide, tert-butylperoxy acetate, tert-butylperoxy benzoate, tert-butylperoxy octoate, tert-butylperoxy neodecanoate, tert-butylperoxy isobutarate, lauroyl peroxide, tert-amylperoxy pivalate, tert-butylperoxy pivalate, dicumyl peroxide, benzoyl peroxide, potassium persulfate, ammonium persulfate,
azo compounds, such as: 2-2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(2-butanenitrile), 4,4xe2x80x2-azobis(4-pentanoic acid), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), 2-(tert-butylazo)-2-cyanopropane, 2,2xe2x80x2-azobis[2-methyl-N-(1,1)-bis(hydroxymethyl)-2-hydroxyethyl]propionamide, 2,2xe2x80x2-azobis[2-methyl-N-hydroxyethyl]propionamide, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine) dichloride, 2,2xe2x80x2-azobis(2-amidinopropane) dichloride, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramide), 5 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide), 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide), 2,2xe2x80x2-azobis[2-methyl-N-(2-hydroxyethyl)propionamide)], 2,2xe2x80x2-azobis(isobutyramide) dihydrate,
redox systems including combinations such as:
mixtures of hydrogen peroxide, alkyl peroxide, peresters, percarbonates, and the like, and any one of the salts of iron, titanous salts, zinc formaldehyde-sulfoxylate, or sodium formaldehyde-sulfoxylate, and reducing sugars,
persulfates, perborate, or perchlorate of alkali metals or of ammonium, combined with a bisulfite of an alkali metal, such as sodium metabisulfite, and reducing sugars,
persulfate of an alkali metal combined with an arylphosphinic acid, such as benzenephosphonic acid and like compounds, and reducing sugars.
The amount of initiator to be used is generally calculated so that the amount of radicals generated, as a ratio to the amount of compound (II), is at most 20 mol %, preferably at most 5%.
According to the process of the invention, the ethylenically unsaturated monomers are more specifically selected among styrene or its derivatives, butadiene, chloroprene, (meth)acrylic esters, vinyl esters and vinyl nitrites. (Meth)acrylic esters denote the esters of acrylic acid and of methacrylic acid with hydrogenated or fluorinated C1-C12 alcohols, preferably C1-C8 alcohols. Among compounds of this type mention may be made of: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, tert-butyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate.
Vinyl nitrites include more particularly those having from 3 to 12 carbon atoms, such as, in particular, acrylonitrile and methacrylonitrile.
It should be noted that styrene may be partially or completely replaced by derivatives, such as alpha-methylstyrene or vinyltoluene.
Particular other ethylenically unsaturated monomers which may be used, alone or as a mixture, or which may be copolymerized with the above monomers, are:
vinyl esters of a carboxylic acid, e.g. vinyl acetate, vinyl Versatate(copyright), vinyl propionate,
vinyl halides,
ethylenic unsaturated mono- and dicarboxylic acids, e.g. acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and the monoalkyl esters of dicarboxylic acids of the type mentioned with alkanols preferably having from 1 to 4 carbon atoms, and their N-substituted derivatives,
amides of unsaturated carboxylic acids, e.g. acrylamide, methacrylamide, N-methylolacrylamide, or methacrylamide, N-alkylacrslamides.
ethylenic monomers containing a sulfonic acid group and their alkali metal or ammonium salts, for example vinylsulfonic acid, vinylbenzenesulfonic acid, alpha-acrylamidomethylpropanesulfonic acid, 2-sulfoethylene methacrylate,
amides of vinylamine, particularly vinylformamide or vinylacetamide,
unsaturated ethylenic monomers containing a secondary, tertiary, or quaternary amino group, or a heterocyclic group containing nitrogen, for example vinylpyridines, vinylimidazole, aminoalkyl (meth)acrylates, and aminoalkyl(meth)acrylamides, e.g. dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, di-tert-butylaminoethyl acrylate, di-tert-butylaminoethyl methacrylate, dimethylaminomethylacrylamide, or -methacrylamide. It is equally possible to use zwitterionic monomers, for example sulfopropyl(dimethyl)aminopropyl acrylate.
To prepare polyvinylamines, the ethylenically unsaturated monomers used are preferably amides of vinylamine, for example vinylformamide or vinylacetamide. The polymer obtained is then hydrolyzed, the pH being acidic or basic.
To prepare polyvinyl alcohols, the ethylenically unsaturated monomers used are preferably vinyl esters of carboxylic acid, for example vinyl acetate. The polymer obtained is then hydrolyzed, the pH being acidic or basic.
The types and amounts of polymerizable monomers used according to the present invention vary as a function of the particular final application for which the polymer is destined. These variations are well known and can readily be calculated by the skilled worker.
The polymerization may be carried out in bulk, in solution, or in emulsion. It is preferably implemented in emulsion.
The process is preferably implemented semi-continuously.
The temperature may vary between ambient temperature and 150xc2x0 C., according to the nature of the monomers used.
The instantaneous polymer content as a ratio of the instantaneous amount of monomer and of polymer during the polymerization is generally between 50 and 99% by weight, preferably between 75 and 99%, still more preferably between 90 and 99%. This content is maintained in a known manner, via control of the temperature, of the addition rate of the reactants, and, optionally, of the polymerization initiator.
The process is generally implemented in the absence of any UV source.
The process of the invention has the advantage of allowing control of the number-average molecular masses Mn of the polymers. Thus these masses Mn are close to the theoretical values Mn th, where Mn th is given by the following formula       M    nth    =                              [          M          ]                0                              [          P          ]                0              ⁢          X      100        ⁢          M      0      
in which:
[M]0 represents the initial molar concentration of monomer
[P]0 represents the initial concentration of precursor compound
X represents the monomer conversion expressed as a percentage
M0 represents the molar mass of the monomer (g/mol).
According to the present invention, the control of Mn is particularly apparent at the start of the polymerization.
In addition, the polymerization process according to the present invention leads to polymers with a low polydispersity index (Ip=Mw/Mn, where Mw: weight-average molecular mass), close to 1.
The invention therefore also provides polymers obtainable by the process consisting of bringing at least one ethylenically unsaturated monomer into contact with at least one source of free radicals and at least one compound of formula (IA), (IB), or (IC).
The polymers generally have a polydispersity index of at most 2, preferably of at most 1.5.
The invention also provides a process for preparing multiblock polymers, in which the implementation of the polymerization process described above is repeated at least once, using:
compared with the preceding implementation, different monomers, and
instead of the compound (I) of formula (IA), (IB), or (IC), the polymer from the preceding implementation, known as a precursor polymer.
The complete process for synthesizing a block polymer according to the invention may therefore consist in:
(1) synthesizing a precursor polymer by ringing into contact an ethylenically unsaturated monomer, a source of free radicals, and a compound of formula (IA), (IB), or (IC),
(2) using the precursor polymer obtained in step (1) to prepare a diblock polymer by bringing this precursor polymer into contact with a new ethylenically unsaturated monomer and a source of free radicals.
This step (2) may be repeated as many times as desired with new monomers, to synthesize new blocks and obtain a multiblock polymer.
If the implementation is repeated once, a triblock polymer will be obtained, and if it is repeated a second time, a xe2x80x9cquadriblockxe2x80x9d polymer will be obtained, and so on. With each fresh implementation, therefore, the product obtained is a block polymer having an additional polymer block.
To prepare multiblock polymers, therefore, the process consists in repeating the implementation of the preceding process a number of times on the block polymer coming from each preceding implementation, with different monomers.
The compounds of formula (IB) and (IC) are particularly interesting because they allow a polymer chain to be grown at at least two active sites. With compounds such as these it is possible to economize on polymerization steps to obtain a copolymer of n blocks. Thus, if the value of p is 2 in the formula (IB) or (IC), the first block is obtained by polymerizing a monomer M1 in the presence of the compound of formula (IB) or (IC). This first block may then grow at each of its extremities via polymerization of a second monomer M2. A triblock copolymer is obtained, and this triblock polymer itself can grow at each of its extremities via polymerization of a third monomer M3. Thus, a xe2x80x9cpentablockxe2x80x9d copolymer is obtained in only three steps. If p is greater than 2, the process can give homopolymers or block copolymers whose structure is
xe2x80x9cmulti-branchedxe2x80x9d or hyperbranched.
According to this process for preparing multiblock polymers, if it is desired that the block polymers obtained are homogeneous and do not have a composition gradient, and if all of the successive polymerizations are carried out in the same reactor, it is essential that all the monomers used in one step are consumed before the polymerization of the next step starts, e.g. before the new monomers are introduced.
As for the process for polymerizing a monoblock polymer, this process for polymerizing block polymers has the advantage of leading to block polymers having a low polydispersity index. It also allows control of the molecular mass of block polymers.
The invention therefore provides block polymers obtainable by the preceding process.
These block polymers generally have a polydispersity index of at most 2, preferably of at most 1.5.
The invention particularly provides block polymers which have at least two polymer blocks selected among the following partners:
polystyrene/polymethyl acrylate
polystyrene/polyethyl acrylate,
polystyrene/polytert-butyl acrylate,
polyethyl acrylate/polyvinyl acetate,
polybutyl acrylate/polyvinyl acetate
polytert-butyl acrylate/polyvinyl acetate.
When use is made of compounds of formula (IA), the block polymers obtained have a structure of the type: 
in which:
R2, R1 are as defined above,
V, Vxe2x80x2, W and Wxe2x80x2 are identical or different and represent: H, an alkyl group, or a halogen,
X, Xxe2x80x2, Y, and Yxe2x80x2 are identical or different and represent H, a halogen, or an R3, OR3, O2COR3, NHCOH, OH, NH2, NHR3, N(R3)2, (R3)2N+Oxe2x88x92, NHCOR3, CO2H, CO2R3, CN, CONH2, CONHR3 or CONR32 group, in which R3 is selected among alkyl, aryl, aralkyl, alkaryl, alkene, or organosilyl groups, optionally perfluorinated, and optinally substituted by one or more carboxy, epoxy, hydroxyl, alkoxy, amino, halogen, or sulfonic groups,
a and b are identical or different and have values 0 or 1,
m and n are identical or different and are greater than or equal to 1, and if one of these is greater than 1, the repeat units are identical or different.
These block polymers are the result of bringing into contact:
an ethylenically unsaturated monomer of formula: CYYxe2x80x2 (=CW-CWxe2x80x2)b=CH2,
a precursor polymer of general formula (IIA): 
a source of free radicals.
The polymer (IIA) is the result of bringing into contact an ethylenically unsaturated monomer of formula: CXXxe2x80x2 (=CV-CVxe2x80x2)a=CH2, a compound (I) of general formula (IA) and a source of free radicals.
In the formula (IIA), n is preferably greater than or equal to 6.
Particularly preferred compounds (IIA) are homopolymers of styrene (Yxe2x80x2=H, Y=C6H5, b=0), of methyl acrylate (Yxe2x80x2=H, Y=COOMe, b=0), of ethyl acrylate (Yxe2x80x2=H, Y=COOEt, b=0), of butyl acrylate (Yxe2x80x2=H, Y=COOBu, b=0), of tert-butyl acrylate (Yxe2x80x2=H. Y=COOtBu, b=0), of vinyl acetate (Yxe2x80x2=H, Y=OCOMe, b=0), of acrylic acid (Yxe2x80x2=H, Y=COOH, b=0), and for which:
R1xe2x95x90CHCH3(CO2Et), CH(CO2Et)2, or C(CH3)2(C6H5), and
R2xe2x95x90xe2x80x94CH2CF3, xe2x80x94CH2CF2CF2CF3, or CH2CH2C6F13.